The present invention relates to polarimetric synthetic aperture radar. The synthetic aperture radar image is a two-dimensional image, with the two dimensions corresponding to cross-range (azimuth or travel direction) and slant-range (or lateral); each being perpendicular to each other. Synthetic aperture radar frequently uses a platform and the synthetic aperture correlates to the distance the SAR platform covers during the period in which a target can be observed from the forward extent of the azimuth view angle on the platform's approach to the aft extent of the azimuth view angle upon its departure. In addition, the electromagnetic radiation produced by synthetic aperture radar has a polarization. Such polarization is useful for identification of materials. Symmetric, man-made objects produce very different synthetic aperture radar (SAR) signatures when examined using different polarizations. This is especially noticeable when these objects are metal. By using radar pulses with a mixture of polarizations and receiving antennas with a specific polarization, different images can be collected from the same series of radar pulses.
The present invention also relates to detection of manmade objects. Since manmade objects often exhibit “left-right” symmetry not found in nature, sensors capable of detecting “left-right” symmetry have the capability of distinguishing manmade (symmetric) objects from naturally occurring (asymmetric) objects. Detection of large manmade or symmetrical objects using SAR imagery provided the radar aperture is in alignment with the longitudinal axis of the target is disclosed in “Applications of Synthetic Aperture Radar(SAR) to Unexploded Ordnance (UXO) Delineation,” by Janet E. Simms, Duke University, ERDC/GSL TR-03-15, U.S. Army Corps of Engineers, (August 2003) (hereby incorporated by reference herein as though fully reproduced) with the caveat that naturally occurring clutter resulting from foliage resulted in false alarms and was problematic (pages 39 and 46, FIG. 12).
Detection of objects with specific sizes and shapes is disclosed in U.S. Pat. No.8,125,370 ('370 Patent) to Rodgers, et al, hereby incorporated by reference. The '370 Patent discloses a method for processing a polarimetric synthetic aperture radar (SAR) image of a region in order to screen large areas to identify candidate pixels that correspond to a position in the image that contains a candidate object. To obtain polarimetric SAR images, the system disclosed in the '370 Patent transmits and receives pulses with both horizontal and vertical polarization. Polarimetric SAR imagery consists of two, three or four independent channels of complex data (amplitude plus phase) consisting of HH (Horizontal transmit, Horizontal receive), HV (Horizontal transmit, Vertical receive), VV (Vertical transmit, Vertical receive), and VH (Vertical transmit, Horizontal receive). For a fully polarimetric or quad-polarization SAR system (four channels), all four combinations HH, HV, VV and VH are employed.
Generally waves can be resolved into two orthogonal components (linearly, circularly, or elliptically polarized) in the plane transverse to the direction of propagation. See, Wolfgang-Martin Boehner, “BASIC CONCEPTS IN RADAR POLARIMETRY” POLSARPRO V3.0—LECTURE NOTES, Dec. 17, 1999, http://earth.esa.int/landtraining07/polsar_basic_concepts.pdf., at page 12. The processing of different polarizations is particularly useful when metal objects are encountered. For example, a co-pol (HH or VV) response will be very high in the pixels containing a metal object's points of “left-right” symmetry. This could include multiple downrange pixels, depending on the target size and image pixel size. Such pixels could represent, for example, the centers of unexploded ordinances, and the centers of trihedrals or similar “corner-like” structures. The cross-pol response, on the other hand, will be very small in the same image pixels. It is an object of the present invention to exploit this physical phenomenon to enhance the target response from symmetric, man-made objects.